In a large saucepan, combine blueberries, sugar and lemon juice. Stir well and bring the mixture to a boil, pressing down on the berries to extract the juice. Reduce the heat to medium-low and simmer for 3 minutes while continuing to mash the berries.
Remove from the heat and strain the mixture through a fine sieve sitting over a large bowl, pressing down to extract as much liquid as possible; discard the pulp.
In another saucepan over medium heat, add heavy cream and milk. Stir well and bring to a gentle simmer or until it reaches 140ºF.
In a medium bowl, add the beaten eggs. By the ladle and in a slow stream, temper the eggs with the hot heavy cream, whisking constantly.
When the yolks are tempered, add them to the saucepan, whisking constantly, and cook for 2 minutes or until the custard reaches 170ºF.
Remove from the heat and pass it through a fine sieve sitting over the blueberry bowl; discard any cooked eggs if there are any.
Add vanilla extract and a small pinch of fleur de sel to the mixture; whisk until blended.
Place a piece of wax paper on top making sure it touches the surface to prevent a skin from forming and let it cool off for 30 minutes before transferring to the fridge; chill for 5 hours but preferably overnight.
When ready to churn, pour chilled custard into the bowl of an ice cream machine and process according to the manufacturers’ instructions until it reaches a desired consistency.
Spoon the mixture into a 2-quart container, place the lid on and transfer to the freezer until frozen, about 4 hours or overnight.